The developmental mutants of Drosophila melanogaster have proven to be a rich and productive area of study for investigations into basic mechanisms in development. We have chosen one such genetic locus, engrailed for intensive study. Our work and the work of others has shown that the engrailed locus is essential in normal morphogenesis of all of the developing segments of Drosophila and that this requirement is restricted to the cells of the posterior compartment in each segment. Our work has more recently revealed that the locus is extremely large and complex: engrailed mutations define a region that exceeds 70,000 base pairs, interactions between different mutant alleles are complex, and the RNA transcripts that the locus produces are many and vary with respect to their time of expression, their size, their sequence composition, and the cell type in which they are expressed. How the locus is regulated to function only in posterior compartment cells and how the locus functions to restrict the posterior compartment cells to grow only within the borders of the posterior compartments are two basic questions that remain intriguing but unanswered. We will approach these questions by 1) correlating specific phenotypes of different engrailed mutants with specific RNA transcripts from the locus; 2) defining the controlling sequence elements responsible for position and cell type specific expression of these RNA transcripts; and 3) identifying and characterizing the protein products of the locus.